add the ability to have subslice parser combinators

[herkhinah]

I want to be able to implement parser combinators where the second parser parses only the input slice the previous parser has parsed. E.g. an AndIs implementation where the second parser is limited to the input the first parser has parsed.

As I'm making changes that feel more to the core than my previous I wanted to ask if I'm on the right track implementation wise and if such functionality would be welcome.

I've added a (badly named) trait Slice that is a SliceInput with the additional constraint Self = SliceInput::Slice. The use case is for parser combinators like the one described above so that both parser can have the same error type. Then there is the with_subslice function that limits the input in InputRef for the passed closure to the passed range.

[zesterer]

We need to be a bit careful here: offset is used for prioritising errors, and this effectively ends up lying to the rest of the crate about the offset of an error, which could have strange ramifications.

[zesterer]

I think I'm a bit uncomfortable about merging this until we have a specific use-case for it (such as AndIs, as you say). It's likely that this code will need changing a little for such a case anyway, so I'd prefer to merge it along with code that actually uses it and hence can be tested.

[herkhinah]

Yes, it was a hack for a use case that I don't have anymore.